In the communications industry there are many applications wherein it is necessary to transmit a wide frequency band of information or signals while virtually eliminating, or trapping, a narrow frequency band of signals. A type of filter for this purpose is known as a stop band filter. This filter is also known as a notch filter because of its well-known "plateau and valley" transfer characteristic. Such a characteristic, of course, is generally flat (the plateau portion) over all frequencies to be transmitted except that narrow frequency band to be eliminated, and it is here that the transfer characteristic falls off rapidly (the valley portion) on both sides of the center frequency (f.sub.c) that identifies the center of the stop band, or notch.
This type of filter is particularly attractive for applications involving video or television broadcast signals where very wide passband bandwidths are needed. These video channels may include narrow bandwidth modulated carrier signals that must be trapped out to prevent video distortion. In addition, there are situations where interference may be caused by adjacent channel carrier frequencies or narrow band communication signals. A typical application for such trap filters is in CATV systems, wherein it is often necessary to transmit (or distribute) many broadcast channels in the presence of narrow-band noise such as an interfering signal.
Another notch filter application is to eliminate a carrier frequency signal that is used to scramble a television signal. The carrier would be inserted between the sidebands of the typical broadcast television video signal that are located at multiples of 15,750 Hz from the carrier. The filter must have minimum loss at the adjacent sidebands and should have a pass band extending across the VHF broadcast band.
It is known that one measure of quality of such notch filters is the ratio of the pass band--the frequency range or band of signals that are transmitted--to the stop band--the frequency band of signals blocked or trapped by the filter.
It is further known that quartz crystals are well suited for implementing such filters, due to their small size, excellent frequency stability, and low manufacturing costs. These crystals are passive components, and thus require no power. Moreover, a crystal notch filter with a high pass band-to-stop band ratio is very desirable for communication applications. In the past, ladder circuits using inductors in the series arms and crystals in the shunt arms have been used for such filters. However, these ladder circuits do not provide the wide pass band region that is desired. As a result, there is a need for an improved crystal notch filter.